The low pressure production process of polyethylene using a catalyst comprising an organomagnesium compound and a transition metal compound is already publicly known in the patent of K. Ziegler (Japanese Patent Publication No. 1546/1957). However, organomagnesium compound itself is insoluble in the inert hydrocarbon media used for the synthesis of catalyst and the polymerization reaction, so that it has never been used effectively and no success has ever been achieved in obtaining a high activity therefrom.
As catalysts of which activity is enhanced by using an organomagnesium compound in a specific form, there are known systems using, for example, an ether complex of organomagnesium halogenide, the so-called Grignard reagent, or one using organomagnesium alkoxide (Japanese Patent Publication No. 40,959/1972; Japanese Patent Kokai (Laid-Open) No. 19,274/1971). Though these catalysts exhibit a considerably high activity per atom of transition metal, they are still insufficient to omit the catalyst removal step of the polyethylene production process completely in that the halogen remains in the liquid reaction mixture to such an extent as being not disregarded.
The present inventors had already discovered a series of catalyst systems using a complex compound soluble in inert hydrocarbons. These catalysts systems comprise an organomagnesium compound together with an organoaluminum compound, an organozinc compound, an organoboron compound and an organoberyllium compound, respectively. (U.S. Pat. Nos. 3,989,878, 4,004,071, 4,027,087). These catalyst systems have a much higher activity than the above-mentioned disclosed catalysts and enable to omit the catalyst removal step completely.